The "Use cache for histograms" check box tells Photoshop whether to generate the histograms that appear in the Levels and Threshold dialog boxes based on the cached sampling or the original image. As I explain in Chapter 17, a histogram is a bar graph of the colors in an image. When you choose a command such as Image ^ Adjust ^ Levels, Photoshop must spend a few seconds graphing the colors. If you turn the Use Cache for Histograms check box on, Photoshop graphs the colors in the reduced screen view, which takes less time, but is also less accurate. Turn the check box off for slower, more accurate histograms.
Generally speaking, I say leave the option on. A histogram is merely a visual indicator and most folks are unable to judge the difference between a downsampled histogram and a fully accurate one.
Again, if you're working in very large images and you have the Cache Levels value maxed out at 8, you should probably leave this check box selected. But if you have to reduce the Cache Levels value, turn off the check box. Histograms are the first thing that can go.
Note This option is not responsible for the histogram irregularities that popped up in
Photoshop 4. The fact that the Threshold dialog box sometimes lifted its histogram from the active layer only was a bug, not a function of Use cache for histograms. Even so, this option has received a lot of flack it did not deserve. My opinion is that, on balance, this is a positive feature that should be left on.
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